This invention relates generally to the field of mass flow measurement in fluids, particularly in fluid flows in ducts where the fluid velocity varies substantially throughout a transverse plane across the duct.
The individual sensors making up the whole of the device can generally be referred to as a type of thermal anemometer which is a well known method of determining mass flow by measuring the electrical energy being dissipated in a heated element which is exposed to the flow and is exactly balancing the heat transfer to the flow due to its increased temperature. Examples of prior art in thermal anemometer sensors and corresponding circuits are shown in U.S. Pat. Nos. 3,138,025, 3,333,470, 3,352,154, 3,604,261, 3,900,819, 4,024,761, and 4,206,638 as discussed by Djorjup (U.S. Pat. No. 4,279,147). This basic device has been improved upon by several inventors. Peter (U.S. Pat. No. 4,213,335), for example, shows an improvement by placing the active sensors of his anemometer so that they are substantially parallel to the streamlines of the flow into which they are placed. The major improvement is to protect the sensors from dirt build up which significantly alters calibration.
My invention is also concerned with the placement of a plurality of sensors (thermistors) in a duct in such a fashion as to yield, through electrical means, a signal which is representative of the true mean velocity of the fluid medium. While not specifically discussed by Peter, others have considered the placement of an array or continuous string of sensors in a duct to provide sufficient coverage. Tatum (U.S. Pat. No. 1,240,797) teaches the usefulness of disposing a temperature dependent resistor uniformly throughout the conduit to provide an average temperature. It should be pointed out that temperature dependent resistors are commonly found with linear transfer functions supporting the accuracy of this concept. However, the average flow speed obtained with Tatum's U.S. Pat. No. 1,240,797 device would be valid only when the flow within the conduit is nearly uniform. Although Tatum discusses rectangular support members within a circular duct, it is noted that such supports as shown in the embodiments discussed below located in a rectangular duct would follow easily from what he teaches. Similarly, Webb (U.S. Pat. No. 3,472,080) discusses the use of separate vertical and horizontal members made up of flow sensitive temperature dependent resistors aligned to form a grid work and produce an overall average velocity of the flow.
In these known devices, especially in the case where a plurality of sensors are used to reveal details of the flow profile in a duct, the outputed "average" flow within a duct is subject to substantial errors due to the inherent non-linear transfer function associated with these types of prior art devices. The error obtained by a linear algebraic average of the outputs of several "non-linear" sensors as proposed by others depend on the non-uniformity of the flow field passing over the sensor array but can be substantial in mildly contorted flows.
The thermistors discussed in this disclosure are subject to the same shifts in calibration as are expected in other thermal anemometers when the ambient temperature of the fluid medium passing the sensors changes. Djorup (U.S. Pat. No. 4,279,147) teaches the usefullness of placing one active sensor pointing in a direction 180.degree. away from another sensor and using the difference in the output from the two sensors to indicate flow direction. In my invention, the difference between sensors exposed to the flowing medium and ones shielded entirely from the flow are used to correct for changes in the temperature of the ambient medium.